Glycosyl transferases are group of enzymes which are associated with the cell membrane and have been implicated to play a role for intercellular adhesion and tumor invasion. We are studying one of these enzymes namely N-acetyglucosaminide-Beta1-4 galactosyl transferase (GT). This enzyme beside transferring galactose from UDP-galactose to nonreducing terminal residue N-acetyl glucosamine in glycoproteins, is also modified by milk specific protein Alpha-lactalbumin to transfer galactose to glucose, for the synthesize of lactorse in mammary gland. GT was purified from rat milk by affinity chromatography on N-acetylglucosamine-sepharose and Alpha-lactalbumin-sepharose columns. The purified enzyme from rat milk showed three polypeptides of Mr 59k, 54, and 27k. GT purified from human milk under similar conditions. Was electrophoretically homogenous showing one polypeptide of Mr 54k. Rat and human milk GT differed in its substrate constants besides being antigenically different. Since glycosyl transferases are involved in transferring the carbohydrate moieties onto the cell surface glycogconjugates and cell surface carbohydrates are known to alter during embryogenesis, we studied the changes in cell surface GT in mouse embryonal carcinoma cells (F9) upon differentiation with retinoic acid and cyclic AMP. There was an increase in activity with the treatment and this increase could be blocked by either actinomycin D or cycloheximide.